It is known that hydrogen peroxide is a moderately effective, mild disinfecting agent with bacteriostatic properties. Whereas H.sub.2 O.sub.2 concentrations of 25 mg/l do inhibit the growth of some bacteria, an effective lowering of the bacterial count requires many hours, even at much higher H.sub.2 O.sub.2 concentrations, or an additional UV irradiation process. Such measures are inadequate and, in addition, not economical for disinfecting large quantities of water, for instance the treatment of water in clarification plants and their outlets.
Peroxyacetic acid (PAA), on the other hand, is a highly effective active agent, the use of which enables a rapid reduction of bacteria. Peracetic acid is employed, inter alia, for sterilization of items used in the food industry and for the disinfection of bottles and in hospitals. Commercially available peracetic acid solutions with a content of 15 wt. % PAA additionally contain about 14 wt. % H.sub.2 O.sub.2 and 28 wt. % acetic acid. The use of PAA solutions of this type causes, on account of their unpleasant corrosive and fire-accelerating properties, problems with regard to handling, storage, materials and transport. A further disadvantage is the high content of acetic acid, which can result in an increased COD content and, additionally, in a drastic bacterial aftergrowth.
The previously demonstrated disadvantages of the PAA solutions can, according to DE-OS 195 31 241.4, be avoided by using a peracetic acid solution of low concentration with a content of 0.5 to 2.5 wt. % PAA, 40 to 50 wt. % hydrogen peroxide and less than 2 wt. % acetic acid. Despite the acknowledged good effect of such a peracetic acid solution as a disinfecting agent, in some applications there is a demand for a more rapid and, in some cases, a still more efficiently active disinfecting agent based on active-oxygen compounds that does not present the disadvantages of the aforementioned PAA solutions with a PAA content of 15 wt. % or above.
An alternative to disinfecting agents based on solutions containing peracetic acid and hydrogen peroxide is constituted by aqueous solutions that contain performic acid (PFA), PAA, hydrogen peroxide and formic acid. According to J. Hyg. Epidem. Microbiol. Immunol. (1968) 12, 115, performic acid possibly exhibits better fungicidal properties than peracetic acid, but the effect might also be attributed to formic acid. By using a solution of this type it is possible, according to EP 0 231 632 A2, for micro-organisms on surfaces and, according to WO 94/20424, in water circuits to be controlled or destroyed within about 0.5 to 60 minutes at a usage concentration in the range from 1 to 1,000 ppm, in particular 5 to 100 ppm, of performic acid.
The PFA solution according to WO 94/20424 and EP 0 231 632 is produced in situ by reaction of hydrogen peroxide with formic acid, the molar ratio of H.sub.2 O.sub.2 to formic acid lying in the range from 1:10 to 10:1. Highly concentrated formic acid and 20 to 50 wt. % hydrogen peroxide are employed. Disadvantages of such a PFA solution are the not inconsiderable risk of explosion and the high corrosivity and hence problems as regards handling and safety.
From WO 95/24388 it is known that the reaction of formic acid with hydrogen peroxide proceeds fairly slowly until equilibrium is established, but the formation of PFA can be accelerated in the presence of a carboxylic ester, for example ethyl acetate, by way of catalyst. In addition to a carboxylic ester or as an alternative thereto, a compound with a --C(O)--O-- group-- which, however, is not a carboxylic group--may serve as catalyst; in this document, however, there is no suggestion as to why it might be a question of compounds of this type.
A disinfecting agent containing performic acid is also known from WO 95/32625: the performic acid is formed by reaction of a formic ester such as ethyl formate with an oxidant, in particular hydrogen peroxide. Although the diminished corrosivity in comparison with the formic-acid/hydrogen-peroxide system is advantageous, the very low concentration of performic acid as demonstrated by the Examples--as a result of which an adequate rate of kill can only be achieved after a relatively long exposure time--is disadvantageous. In addition, a formic ester is more expensive than formic acid.
In DE-A 39 29 335 a process is taught for the disinfection of textiles in washing installations, whereby an agent containing peroxycarboxylic acid is admixed in the final rinsing operation. This agent is, in particular, an equilibrium peroxycarboxylic acid solution with a content of 40 to 60 wt. % carboxylic acid, 2 to 5 wt. % peroxycarboxylic acid and 0.1 to 3 wt. % hydrogen peroxide. With a view to producing the agent, in addition to acetic acid use may also be made of formic acid and propionic acid, as well as mixtures of such acids, by way of carboxylic acid; no details can be gathered from this document relating to two carboxylic acids and, accordingly, agents containing the corresponding peroxycarboxylic acids.
Although disinfecting agents based on performic acid may in some cases display a higher microbiocidal effect than those based on peracetic acid, the production and handling of solutions containing performic acid in higher concentration is problematic.